CHAPTER Multimedia-Service Creation Overview 5 One of the major benefits that SIP offers is its ability to be used by programmers as a tool for multimedia-service creation. In this chapter, we define what "SIP ser- vices" are. We also discuss the different approaches to provide SIP services, and analyze the tools available for SIP-service creation. 5.1 What are SIP Services? As we saw in the Chapter 4, core SIP provides a basic functionality for the man- agement of sessions and the location of users. Such functionality at the signaling level, together with other protocols at the media level, can enable a wide variety of multimedia scenarios. Nevertheless, there are cases where users may want an enhanced function- ality on top of the basic multimedia-session management. For instance, imagine that the called user wants incoming calls from specific users to be blocked, or a company wants all the incoming calls to a group number to be distributed among a group of agents. These are examples of services--that is, of enhanced func- tional-ity on top of the basic multimedia-session-management functions. It is not always the end users who demand a specific enhanced treatment for their calls. For instance, in order to decide whether a call can continue or not, a service provider might want all originating calls from prepaid users to go through a service logic that checks if the user has enough credit in his or her account. The previously described services can be easily implemented by having an extra service logic that implements a smart manipulation of the SIP signaling. This is yet another example of the benefits that the signaling concept offers in commu- nication scenarios: it enables the delivery of services. So far, we have mentioned just scenarios that involve signaling manipulation. However, even the simplest multimedia call requires some media-level handling at the endpoints, at least in order to capture and present the media and to receive and transmit the media packets over the network. Hence, even in the most basic case, there is a need for the applications at the endpoints to have direct access to the media-handling capabilities in the terminal. 59